JPS6361641B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a compact zoom lens, and more particularly to a compact zoom lens with a short overall lens length (distance from the first lens surface to the image plane) that is effective for lens shutter cameras, video cameras, and the like. In recent years, with the miniaturization of lens shutter cameras, video cameras, etc., there has been a demand for zoom lenses with short overall lens lengths. Also, in the field of cameras such as lens-shutter cameras in which lenses are not replaced, there is a demand for a zoom lens, and there is a demand for a compact zoom lens with a length comparable to that of conventional single-focal-length lenses. In Japanese Patent Application Laid-Open No. 57-201213, the applicant
As shown in Figure 1, it is a compact lens that consists of two lens groups, the first lens group with positive refractive power and the second lens group with negative refractive power, and zooms by changing the distance between both lens groups. We proposed a two-group zoom lens. In this proposal, by configuring lens groups with positive and negative refractive powers in order from the object side, a compact zoom lens with a short back focus and a mechanically simple configuration was achieved. Furthermore, in Japanese Patent Application Laid-Open No. 58-184916, it is composed of three lens groups, first, second, and third lens groups having positive, positive, and negative refractive powers in order from the object side, and the three lens groups are moved. A three-group zoom lens that performs magnification change has been proposed. However, with the aforementioned two-group zoom lens, since there are only two lens groups that move during zooming, it is difficult to obtain a predetermined zoom ratio without moving the lens groups relatively often. Since three lens groups are moved to change the magnification, they tend to be mechanically complex. Furthermore, since both zoom lenses also move the aperture when changing the magnification, the movement mechanism tends to become complicated. SUMMARY OF THE INVENTION An object of the present invention is to provide a compact zoom lens that can easily obtain a predetermined variable power ratio, is mechanically simple, and has a short overall lens length. A further object of the present invention is to provide a compact zoom lens suitable for a lens shutter camera with a variable magnification ratio of about 1.5 to 2.0. The main feature of the compact zoom lens for achieving the object of the present invention is that it has three lens groups, first, second, and third lens groups having positive, positive, and negative refractive powers in order from the object side. When changing the magnification from the wide-angle end to the telephoto end zoom position by fixing the second lens group and moving the first lens group and the third lens group toward the object side, the second lens group and the third lens group move toward the object side. Each lens group is configured so that the imaging action of the third lens group is multiplication. Next, FIG. 2 shows a schematic diagram of an optical arrangement according to an embodiment of the present invention. In the figure, , , are the first, second, and third lens groups, respectively, and the arrows indicate the state of movement during zooming. As mentioned above, in the present invention, zooming is achieved mechanically by moving two of the three lens groups, the first and third lens groups, and fixing the second lens group, which often includes an aperture. We are trying to simplify the process. In addition, when changing the zoom position from the wide-angle end to the telephoto end, the lateral magnification β ₂ of the second lens group and the lateral magnification β ₃ of the third lens group are both continuously multiplied. are being carried out efficiently. Furthermore, in the present invention, it is preferable that the combined refractive index of the second lens group and the third lens group is negative, and the lens structure is such that the negative refractive power gradually becomes stronger during zooming. A predetermined zoom ratio is efficiently achieved with a small amount of movement. The compact zoom lens which is the object of the present invention can be achieved by satisfying the above conditions, but it is more preferable to set the above conditions as follows. In the above-mentioned imaging action, if the lateral magnifications at the wide-angle end and the telephoto end are respectively β _2W and β _2T for the second lens group, and β _3W and β _3T for the third lens group, then 1.0<β _2T /β _2W <1.5... (1) 1.0< _β3T / _β3W <1.7…(2) It is to be set within the range. This results in 3
A predetermined zoom ratio can be obtained in a well-balanced manner while reducing aberration fluctuations due to zooming of the two lens groups. If the upper limits of conditional expressions (1) and (2) are exceeded, the zoom ratio becomes too large, the refractive power of each lens group becomes excessive, or the amount of movement of the lens groups becomes excessive, making it difficult to reduce aberration fluctuations due to zooming. becomes difficult. Furthermore, it is optically very difficult to construct a zoom lens that exceeds the lower limits of conditional expressions (1) and (2). In particular, the multiplication ratio is so small that it takes 2
It becomes difficult to move two lens groups. Furthermore, in the present invention, under the conditional expressions (1) and (2), the composite refractive powers of the second lens group and the third lens group at the wide-angle end and telephoto end zoom positions are respectively _23W and _23T .
When 1.2< _23T / _23W <4.0...it is preferable to set it within the range of (3). With such a refractive power arrangement, a predetermined variable power ratio can be easily obtained without increasing the amount of movement of the third lens group. This is because the focal lengths of the second and third lens groups are f ₂ and f ₃ respectively, the distance between the principal points of the second and third lens groups is e ₂ , and the composition of the second and third lens groups is If the refractive power is ₂₃ , it can be expressed as ₂₃ = 1/f ₂ + 1/f ₃ −e ₂ /f ₂ f ₃ (a). Here, in the above formula (a), 1/f ₂ >0, 1/f ₃ <0, so 1/f ₂ +1/f ₃ <0
By setting the focal length of each lens group so that e ₂ becomes smaller when changing the zoom position from the wide-angle end to the telephoto end, that is, by moving the third lens group toward the object side, the refractive power ₂₃ is gradually increased. This is because it can be made stronger in the negative direction. Therefore, the refractive power ₂₃ at the telephoto end zoom position can be maximized in the negative direction, and as a result, a high zoom ratio can be easily obtained by slightly moving the third lens group. . As explained above, the object of the present invention can be satisfactorily achieved by selecting the configuration of each lens group, but in order to achieve even better aberration correction over the entire zoom range and to reduce the size of the zoom lens, the following steps are required: It is preferable to satisfy the following conditions. The focal lengths of the first and third lens groups are f ₁ and
f ₃ , the focal length of the entire system at the wide-angle end zoom position is f _W , the distance between the principal points of the first lens group and the second lens group at the wide-angle end zoom position is e _12W , and at the telephoto end zoom position When the principal point distance between the second lens group and the third lens group is e _23T , 0.3f _W <f ₁ <5f _W ...(4) −2.5f _W <f ₃ <−0.5f _W ……(5) e _12W ＞−0.5f _W ……(6) e _23T ＞−0.4f _W ……(7) The following conditions are satisfied. Conditional expression (4) relates to the refractive power of the first lens group, and when the lower limit is exceeded, the refractive powers of the second and third lens groups must have large negative values, especially at the telephoto end, in order to obtain a variable magnification ratio. Therefore, the Petzval sum increases in the negative direction and the curvature of field becomes excessive. On the other hand, if the upper limit is exceeded, not only will the contribution of zooming to the first lens group become smaller, but the lateral magnification _β3 of the third lens group will have to be reduced, and the negative refractive power of the third lens group will be reduced. It has to be made smaller. As a result, the amount of movement of the third lens group increases, and the overall length of the lens increases, which is not appropriate. Conditional expression (5) relates to the refractive power of the third lens group, and when the upper limit is exceeded, the negative refractive power becomes large and the Petzval sum increases in the negative direction, so the curvature of field increases in the positive direction. If the lower limit is exceeded, the combined refractive power with the second lens group approaches the positive direction, so as mentioned above, a high zoom ratio cannot be obtained by slightly moving the third lens group, and a large amount of movement is required. Become. Also, if you exceed the lower limit and try to maintain the composite refractive power in the negative direction to obtain a predetermined zoom ratio, then according to equation (a) above, the second
The positive refractive power of the lens group becomes weak, and it is necessary to reduce the principal point distance e ₂ between the second lens group and the third lens group, which is not appropriate because the movement space for the third lens cannot be obtained. Conditional expressions (6) and (7) are intended to shorten the overall length of the lens by appropriately setting the distance between each lens group.
If the ranges of (6) and (7) are exceeded, the distance between the lens groups at the wide-angle end and the telephoto end becomes too narrow, causing interference between the lens groups, which is undesirable in terms of lens construction. In the present invention, the focal length f ₂ of the second lens group, which is fixed during zooming, is set as 0.5f _W < f ₂ < 6f _W (8). This is preferable for promoting the magnification change effect caused by the movement of the lens and also for satisfactorily correcting aberrations of the entire screen during magnification change. If the upper limit of conditional expression (8) is exceeded, the refractive power of the second lens group becomes weaker, and when trying to obtain a predetermined variable power ratio, the second lens group, the third lens group It is necessary to take a large principal point interval e ₂ , and the third
The negative refractive power of the lens group also becomes weaker and the amount of movement becomes larger. If the lower limit is exceeded, the refractive power of the second lens group becomes strong, the negative refractive power of the third lens group becomes excessive, and the distance between the principal points of the second lens group and the third lens group becomes small, resulting in a narrow movement space. It's gone and it's not appropriate. In a zoom lens in which the third lens group moves during zooming as in the present invention, the third lens group is composed of a meniscus-shaped lens with a negative refractive power with a convex surface facing the image surface side, over the entire zooming range. This is preferable for favorably correcting the image plane characteristics of the entire screen. In addition, in a zoom lens in which the first and third lens groups move during zooming as in the present invention, in order to reduce aberration fluctuations due to zooming and to satisfactorily correct various aberrations over the entire screen, it is necessary to The radius of curvature of the first lens surface on the object side of the lens group is R ₁₁ ,
When R ₃₁ , it is preferable to satisfy the following conditions: 0.5f _W <R ₁₁ <f _W (9) −0.6f _W <R ₃₁ <−0.2f _W (10). conditional expression
(9) and (10) are intended to satisfactorily correct the spherical aberration caused by zooming and the curvature of field of the entire screen.If the upper limit of conditional expression (9) and the lower limit of conditional expression (10) are exceeded, spherical aberration occurs. becomes over-corrected, and the curvature of field on the wide-angle side increases in the negative direction. Exceeding the lower limit of conditional expression 9 and the upper limit of conditional expression (10) is not preferable because spherical aberration will be insufficiently corrected and field curvature will increase in the positive direction. In the present invention, it is preferable to make the object-side lens surface of the third lens group an aspherical surface in order to achieve better aberration correction and obtain high-performance optical performance. In the present invention, focusing is preferably performed by extending the entire lens system, but it may also be done by extending the first lens group or the second lens group, or by retracting the third lens group. The second lens group may be extended integrally. As described above, according to the present invention, it is possible to achieve a compact zoom lens that is mechanically simple and has high zooming efficiency. Next, numerical examples of the present invention will be shown. In the numerical examples, R _i is the radius of curvature of the i-th lens surface in order from the object side, D _i is the thickness and air gap of the i-th lens surface in order from the object side, and N _i and ν _i are the radius of curvature of the i-th lens surface in order from the object side. These are the refractive index and Atpe number of the glass of the i-th lens. The aspherical shape has the X-axis in the optical axis direction, the Y-axis in the direction perpendicular to the optical axis, the direction of light travel as positive, the intersection of the vertex of the lens and the X-axis as the origin, and R as the paraxial curvature of the lens surface. Radius, a ₁ , a ₂ , a ₃ , a ₄ , a ₅ , b ₁ , b ₂ , b ₃ , b ₄
When is the aspheric coefficient It is expressed by the following formula. For example, the display "D-03" means "10 ^-3 ".

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【table】 [Brief explanation of drawings]

FIG. 1 is a schematic diagram of an optical system of a conventional two-group zoom lens, FIG. 2 is a schematic diagram of an optical system of an embodiment of the present invention, and FIGS. 3A and B are each a diagram of a numerical embodiment 1 of the present invention. Lens sectional view and various aberration diagrams, FIGS. 4A and 4B are respectively a lens sectional view and various aberration diagrams of Numerical Example 2 of the present invention,
5A and 5B are respectively a lens cross-sectional view and various aberration diagrams of Numerical Example 3 of the present invention, FIGS. 6A and B are lens cross-sectional views and various aberration diagrams of Numerical Example 4 of the present invention, and FIG.
Figures A and B are a lens cross-sectional view and various aberration diagrams of Numerical Example 5 of the present invention, respectively. Figures 8A and B are lens cross-sectional views and various aberration diagrams of Numerical Example 6 of the present invention, respectively, and Figure 9A. , B are a lens sectional view and various aberration diagrams of Numerical Example 7 of the present invention, respectively. In the figure, S and C are sine conditions, △S is a sagittal image,
ΔM is the meridional image plane.

Claims (1)

[Claims] 1. It has three lens groups, first, second, and third lens groups having positive, positive, and negative refractive powers in order from the object side, and the second lens group is fixed, and the By moving the first lens group and the third lens group toward the object side, both the second lens group and the third lens group increase when zooming from the wide-angle end to the telephoto end. As well as changing it so that it doubles,
Each lens group is configured such that the combined refractive power of the second lens group and the third lens group gradually becomes stronger in the negative direction when zooming from the wide-angle end to the telephoto end. zoom lens. 2 The focal lengths of the first and third lens groups are respectively
f ₁ , f ₃ , f _W is the focal length of the entire system at the zoom position at the wide-angle end, e 12W is the distance between the principal points of the first lens group and the second lens group at the zoom position at the wide-angle end, and e _12W is the focal length of the entire system at the zoom position at the wide-angle end. The second lens group and the third lens group at the zoom position
When the distance between the principal points of the lens group is e _23T , the conditional expression is 0.3f _W < f ₁ < 5f _W −2.5f _W < f ₃ < −0.5f _W e _12W > −0.5f _W e _23T > −0.4f _W A compact zoom lens according to claim 1, which satisfies the following.